1. Field of the Invention
The present invention relates to a laser irradiation apparatus used for crystallizing a semiconductor film. In addition, the present invention relates to a method for manufacturing a semiconductor device including a step for crystallizing the semiconductor film using the laser irradiation apparatus.
2. Related Art
A thin film transistor (TFT) using a crystalline semiconductor film has higher mobility by double digits or more than TFT using an amorphous semiconductor film and has an advantage that TFT can be formed on an inexpensive glass substrate by using a laser annealing method.
Lasers are classified broadly into two types of a pulsed laser and a continuous wave laser according to its oscillation system. The output energy of the laser light per unit time, which is peak power, of the pulsed laser is higher by three to six digits than that of the continuous wave laser. Therefore, when the beam spot (the region irradiated with the laser light in fact on the surface of the processing object) is shaped into a rectangle having a length of several cm on a side or into a line having a length of 100 mm or more through an optical system, it is possible to irradiate the semiconductor film efficiently and to enhance the throughput.
Incidentally, the absorption coefficient of the laser light to the semiconductor film depends on the material of the semiconductor film. When a silicon film having a thickness from several tens to several hundreds nm is crystallized with the use of a YAG laser or a YVO4 laser, the second harmonic having a shorter wavelength than the fundamental wave is higher in the absorption coefficient, and therefore the crystallization can be performed efficiently. In addition, in order to emit the harmonic, the fundamental wave is oscillated first, and then it is converted into the harmonic through a harmonic generator or the like. And the fundamental wave and the harmonic are separated as needed (refer to the patent document 1).
[Patent Document 1]
Japanese Patent Unexamined Publication No. 2001-156018
In addition, a solid-state laser has been utilized not only in the laser crystallizing process, but also in the fields of physics and chemistry (for example, a light source of a spectroscopic measuring apparatus), an optical fiber communication, a metal working application (laser welding), and so on.
In the fields of the physics and chemistry and the optical fiber communication, since it is premised to use the laser light at a single wavelength, it is natural to separate the fundamental wave and the harmonic and to use one of them. In the case of the metal working, since the metal absorbs the fundamental wave, the fundamental wave may be used as it is and it is unnecessary to convert it into the harmonic. For these reasons, the solid-state laser available in the market emits the fundamental wave or the harmonic after separating the fundamental wave and the harmonic.
Even in the field of the laser crystallizing apparatus, when the solid-state laser is used in the laser crystallization, it has been common to separate the harmonic which is absorbed in the semiconductor film having solid-phase silicon and to use it singularly, because the fundamental wave is not absorbed in the semiconductor film having solid-phase silicon. In other words, since the semiconductor film does not absorb the emission wavelength (fundamental wave) of the solid-state laser, the second harmonic, the third harmonic, or the higher harmonic, which has a wavelength in the range of the visible light region to the ultraviolet light region, is used in the case of using the solid-state laser in the laser annealing.
However, it is a problem that the energy conversion efficiency for converting the fundamental wave into the harmonic is low. For example, in the case of a Nd: YAG laser, the conversion efficiency from the fundamental wave (wavelength: 1064 nm) to the second harmonic (wavelength: 532 nm) is approximately 50%.
In addition, since the nonlinear optical element for converting into the harmonic has extremely low resistance against the laser light, it is difficult to raise the output of the fundamental wave. For example, a continuous wave YAG laser can output the fundamental wave with an energy as high as 10 kW, while it can output the second harmonic with an energy as low as 10 W in consideration of the resistance of the nonlinear optical element.
Therefore, in order to obtain the necessary energy density for crystallizing the semiconductor film, the area of the beam spot must be narrowed to the size of 10−3 mm2, which is inferior in terms of the throughput. Since the laser light converted into the harmonic has lower energy than the fundamental wave, it is difficult to enhance the throughput by enlarging the area of the beam spot. In particular, since the continuous wave laser has lower output than the pulsed laser per unit time, this tendency is remarkable.
Consequently, it is an object of the present invention to suggest a novel laser annealing method and to provide a method for crystallizing the semiconductor film efficiently. Moreover, it is an object of the present invention to provide a method for manufacturing a semiconductor device having a crystalline semiconductor film formed by the novel laser annealing method.
In view of the above problems, the present inventors found a method for irradiating the laser light to the irradiated object (also referred to as the processing object) without separating the fundamental wave and the harmonic. In other words, the present invention provides a laser irradiation apparatus (laser annealing apparatus) for irradiating the fundamental wave and the wavelength not longer than that of the fundamental wave to the irradiated object wherein the laser light emitted from one resonator (oscillator) having both the fundamental wave and the wavelength not longer than that of the fundamental wave is irradiated simultaneously to the same irradiated surface. In particular, in the present invention, the fundamental wave and the wavelength not longer than that of the fundamental wave are irradiated to the irradiated object without being separated.
As thus described, it is possible to perform the laser irradiation (the laser annealing) efficiently by irradiating the harmonic having low energy and the fundamental wave to the irradiated object without separating them. In other words, since they are irradiated without being separated, the fundamental wave can be irradiated so as to assist the harmonic having low energy. The fundamental wave is preferable in particular because the fundamental wave is easy to be absorbed in the semiconductor film having melted silicon. In other words, the semiconductor film having the melted silicon is high in the absorption coefficient of the fundamental wave compared with the crystalline semiconductor film having solid silicon.
In order to carry out the present invention, the laser irradiation apparatus comprises a laser resonator outputting the fundamental wave and the wavelength not longer than that of the fundamental wave, means for moving the irradiated object and the laser light from the laser resonator relatively, and means for shaping the laser light into linear. Moreover, the means for shaping the laser light into linear has means for converging (focusing) the laser light having the fundamental wave and the wavelength not longer than that of the fundamental wave. In the present invention, the laser light may be generated either in a pulse oscillation or in a continuous wave oscillation.
It is noted that the term “linear” herein used does not mean a line strictly but means a rectangle (or an oblong) having a large aspect ratio. For example, the rectangular shape having an aspect ratio of 2 or more (preferably in the range of 10 to 10000) is referred to as linear though the linear is still included in the rectangular.
A laser medium included in the laser resonator of the present invention is a solid selected from the group consisting of YAG, YLF, YVO4, Y2O3, glass, sapphire, forsterite, LuAg, and LuLiF4, each of which is doped with an ion such as Nd, Yb, Ti, Cr, Ho, or Er. The laser having such a solid as the laser medium is referred to as the solid-state laser.
The wavelength not longer than that of the fundamental wave is for example the harmonic of the fundamental wave, and there are the second harmonic, the third harmonic, the fourth harmonic, and so on. Since the main emission wavelength of the solid-state laser is in the infrared region, the second harmonic is mainly in the visible light region.
As the typical solid-state laser, there are a Nd: YAG laser: 532 nm, a Nd: YVO4 laser: 532 nm, a Nd: YLF laser: 527 nm (or 524 nm), a Ti: Sapphire laser: 345 to 550 nm (the wavelength is variable), and an alexandrite laser: 350 to 410 nm (the wavelength is variable).
In order to convert the fundamental wave into the harmonic, the non-linear optical element such as SHG (Second Harmonic Generation) or THG (Third Harmonic Generation) is used. For example, the crystal whose nonlinear optical constant is relatively large such as KTP (KTiOPO4), BBO (β-BaB2O4), LBO (LiB3O5), CLBO (CsLiB6O10), GdYCOB (GdYCa4O(BO3)3), KDP (KD2PO4), KB5, LiNbO3, Ba2NaNb5O15, or the like is used as the SHG. Particularly, the crystal such as LBO, BBO, KDP, KTP, KB5, CLBO, or the like can increase conversion efficiency from the fundamental wave into the harmonic.
In the present invention, the laser light may be irradiated obliquely to the surface of the processing object. On this occasion, the laser light shaped into linear is irradiated in such a way that the incidence angle φ of the laser light satisfies the inequality φ≧arctan (W/2d) when it is assumed that “W” is the length of the major axis or the minor axis of the linear laser light and that “d” is the thickness of the substrate on which the processing object is deposited and which is translucent to the laser light. When the laser light is thus incident obliquely, it is possible to prevent the interference of the laser light in the processing object and to perform more uniform laser annealing.
The laser irradiation apparatus of the present invention needs only one resonator because the fundamental wave and the harmonic from one resonator are irradiated without being separated. Therefore, the running cost of the resonator can be lowered. In addition, it is easy to adjust the optical system compared with the case in which the laser light having the fundamental wave and the laser light having the harmonic are emitted from the respective resonators and they are combined at the irradiated surface. And since the fundamental wave and the harmonic are shaped into linear through the same optical system, the optical system can be simplified. Of course, the lens and the like for separating the fundamental wave and the harmonic are no longer necessary.
It is considered that when the laser light having the fundamental wave and the harmonic is used to anneal the semiconductor film, the laser light of the fundamental wave is easily absorbed in the semiconductor film because the harmonic can melt the semiconductor film and thereby drastically increasing the absorption coefficient of the fundamental wave to the semiconductor film.
Therefore, it is possible to enlarge the region to be annealed, which is the region having superior crystallinity, by irradiating the laser light without separating the harmonic and the fundamental wave. This can increase the throughput and provide a crystalline semiconductor film of high quality.
With the laser annealing performed as described above, it is possible to manufacture a sophisticated thin film transistor and a semiconductor device having it in the high-throughput process at low cost.